1. Field of the Invention
This invention relates to a coordinate reading apparatus used for inputting position data to a computer, and more particularly to a coordinate reading apparatus which has a coordinate designator, a tablet, and a system for calculating induction signals induced in either the coordinate designator or the tablet due to electric coupling between them, to obtain coordinate values.
2. Description of the Prior Art
Known coordinate reading apparatus generally operate on the basis of electromagnetic induction or electrostatic induction.
The fundamental constitution and operation of the coordinate reading apparatus of the electromagnetic induction type are disclosed in Japanese Patent Publication No. 16317/1984 relating to coordinate reading method and apparatus and in Japanese Patent Publication No. 35069/1984 relating to interpolation system for a coordinate reader.
The coordinate reading apparatus of this kind are capable of reading data even when the coordinate designator is spaced from the tablet by a distance, or height, up to a predetermined value. This height is called "reading height".
Apparatus of this type may be constructed to operate according to the absolute coordinate mode or the relative coordinate mode.
In the "absolute coordinate mode", the position where the coordinate designator is placed is expressed as a coordinate value in a coordinate system which is fixed relative to the tablet, and the coordinate that is read out is referred to as an "absolute coordinate".
In the "relative coordinate mode", when the coordinate designator is once moved away by more than the reading height and is then brought within the reading height, the position initially detected within the reading height is regarded to be an origin, the coordinate value detected thereafter is expressed as an increment from the origin, and the coordinate that is read out is referred to as a "relative coordinate".
The relative coordinate mode is generally used for coordinate reading apparatus to control the cursor of a display unit. FIG. 4(a) shows such a display unit having a display screen 52 on which appears a cursor 53 the movement of which is to be controlled. FIG. 4(b) shows coordinate reading apparatus composed of a tablet 51 and a coordinate designator 50.
When it is desired to move cursor 53 from a point D1 to a point D2 on display screen 52, the operation is carried out as described below.
Coordinate designator 50 is moved from T1 to T11 remaining within the reading height H. By this operation, cursor 53 is moved from D1 to D11.
Coordinate designator 50 is then lifted up above reading height H, and is moved from T11 to T12 which is close to T1. In this case, no new coordinate value is produced, and cursor 53 does not move.
Coordinate designator 50 is moved from T12 to T13 while remaining within reading height H. By this operation, cursor 53 is moved from D11 to D2.
What is characteristic in this operation is that the coordinate designator 50 is lifted up by a distance greater than reading height H and is then returned back to a point T close to the initial position.
The absolute coordinate mode, on the other hand, is often used for operations where a medium to be read, such as a drawing, is placed on the tablet, and points thereon are read out. The medium to be read has a certain thickness and the apparatus is usually so designed that the reading is made as large as possible.
One existing coordinate reading apparatus has been designed to be capable of selecting relative coordinate mode and the absolute coordinate mode. The reading height in such coordinate reading apparatus, however, has generally been set to a slightly large constant value to take into consideration the case where the apparatus will be used in the absolute coordinate mode.
When the conventional coordinate reading apparatus is used in the relative coordinate mode, the coordinate designator must be lifted up by a large distance to return it to the initial position since the reading height has been set to a large value, causing a considerable reduction in operating convenience.
The reading height may be set to a small value if it is simply desired to improve the above-mentioned operability. This, however, would cause the basic performance of the coordinate reading apparatus to be deteriorated.